New and emerging PV devices are often based on the use of lower-cost semiconductors, thin films of conventional semiconductors, and organic-inorganic hybrids. Many of these PV devices are rapidly attaining commercial viability. In some cases, thin film layers are deposited by solution processible methods, which are attractive from a cost and manufacturability perspective. However, all such approaches currently use transparent conducting windows (transparent conductive oxides, TCO) deposited by conventional vacuum-based methods, which sacrifices much of the potential cost and scalability advantages.
Transparent conducting oxides are critical components of both existing and emerging approaches to high-efficiency, low-cost PV devices. TCOs act as electrode elements and diffusion barriers, and their work function influences the open-circuit voltage, thus affecting device efficiency. The basic characteristics of TCO materials include high optical transmissivity across a wide spectrum range and high conductivity. TCO for terrestrial PV applications must also be cost effective. The variety of emerging PV cell types dictates the broad range of TCO material requirements. In many cases, current TCO materials do not satisfy the needs of emerging PV technologies. To maximize the potential of PV devices, it is therefore increasingly important to develop new and improved transparent conductors that are capable of supporting novel PV devices and process-specific requirements. In addition, these materials must have reduced sensitivity to material shortages and have improved scalability.
Three oxides are of major commercial importance today: indium oxide, tin oxide, and zinc oxide. Since the 1960s, tin-doped indium oxide (ITO) has been the most widely used TCO for optoelectronic device applications. Currently, this material offers the best available performance in terms of conductivity and transmissivity. In addition, this material offers excellent environmental stability, reproducibility, and good surface morphology. Not surprisingly, the indium oxide based family of materials currently is the most studied and best-understood TCO. The deposition of ITO in a manufacturing environment is typically done by magnetron sputtering. For high volume use, the cost of sputter targets figures strongly into the cost of the final product. Given the semiprecious nature of indium metal and its price instability (indium metal prices spiked at $900/kg in 2005), there are both economic and technical forces driving the development of alternative TCO materials. While new supplies of indium are available, they cannot be accessed cost-effectively by the mining industry without comprehensive market-driven planning. This means potential price volatility, as has been seen in the past, as sharp moves in demand create short-term price escalations. These factors make it unlikely that ITO can become a commodity product that device manufacturers require. Developing alternative TCO coatings that are composed of less-costly raw materials will eliminate this problem. Tin oxide based TCOs are the most deposited (by volume) today and are used mainly in architectural applications for energy-efficient windows. These windows are deposited by spray pyrolysis. Recent improvements in doped ZnO performance make this material an attractive replacement for ITO in future PV devices. Unfortunately, each major material group has drawbacks. As was mentioned, using indium-based materials as TCO for future PV devices is impeded by economical constraints, such as high indium price and significant price volatility. Zinc-based oxides require doping with Al or Ga. Ga is high price material (˜$500 per kg), and doping with Al requires high degree of control in the O2 sputtering atmosphere, thus challenging the robustness of the chemical composition uniformity across the film. In addition, the demonstrated conductivity of the zinc oxide or tin oxide based materials is inferior to ITO.
TCO films today may be prepared by a number of methods including sputtering, electron beam evaporation, chemical vapor deposition (CVD), pulsed laser deposition, spray pyrolysis, chemical bath and others. Magnetron sputtering is widely used for commercial production of different TCOs. However, this method requires relatively expensive high-vacuum equipment, is energy consuming, and imposes certain limitations on deposition substrate size and throughput. Worth noting, recent engineering advances and market drivers have significantly driven down the cost of sputtering equipment and improved manufacturability. For example, machines, which are capable of coating 1 m×3 m glass sheets from both sides or 60 cm×1 km plastic roll, are offered for 3 million euros. It is widely recognized that sputtering provides the best results in terms of high optical transparency and electrical conductivity of metal-oxide films, particularly ITO, ZnO, and ZnO—Al2O3. Nevertheless, sputtering has a limited ability to control the coating uniformity over large area (as the sputtering targets wear out) and limited capability to control composition of multi-component systems. Hence, compositions should be restrained to 2 components at best. The evaporation method (either thermal or e-beam) has similar drawbacks. The main drawback of producing TCO by metal-organic CVD is the limited availability of volatile precursors with relatively low decomposition temperature. For this reason, vacuum and/or high temperature (400-450° C.) equipment is often needed, which is incompatible with many PV devices. For similar reasons, spray pyrolysis, which is the most used process for tin oxide deposition, cannot be used for direct deposition of TCOs on top of PV structures.
A solution-based process would be economically attractive for TCOs. Recent developments suggested that TCO films can be prepared using solution-based processes such as sol-gel, metal-organic, and nano-powder inks or pastes. However, an economically viable solution-based process, which offers high performance TCO's has not yet been developed. The sol-gel process is inherently slow, requires slow drying and re-heating steps, and high temperature sintering. Therefore, sol-gel processes are incompatible with high-throughput, low-cost processes. For this reason, the sol-gel is often used for the preparation of nano-inks rather than for direct depositions. The metal-organic approach relies on high temperature (450-500° C.) decomposition. Inks and pastes based on nano-powder also require high temperature sintering because of the high degree of porosity of such films. Additionally, the conductivity of solution processed TCO films is usually 1-2 orders of magnitude inferior to that of prepared by sputtering. Thus, although solution-based approaches offer the possibility for large area production, applying these existing methods for PV is often precluded.
An alternative approach to form a transparent conductor for PV application is based on carbon nano-tube (CNT) inks. Currently demonstrated performance (conductivity and transparency) is inferior to traditional ITO or even ZnO. Certain process and material-related issues exist, such as the tendency to aggregate over time in solution. These issues result in high haze films and fixed work function, hence limiting PV applicability. While the attention given to this approach in the scientific community is indicative of an unfulfilled need for a viable solution to the transparent conductor problem, the method itself has not yet provided acceptable performance.
The number of compositions currently used as TCOs is restricted to a few primary and binary systems. This limitation is mainly due to two factors: 1) limited bulk solubility of crystalline metal oxide phases in each other, and 2) technical limitations of currently used processing methods. If these challenges could be overcome, it has been shown that the number of suitable transparent and conductive binary, ternary, and even quaternary phases may be larger. Some may potentially exist in thin films only since the phase separation in this case is kinetically precluded by the film thinness. Low-pressure or high-pressure CVD using solid volatile organometallic precursors is a convenient way to make a large variety of multicomponent TCOs. However, CVD requires high substrate temperature (400-450° C.) for precursor decomposition. Despite the fact that this method can be applied for large area production, it is limited to thermally stable substrates (like glass and metal foils) and cannot be applied for direct TCO layer deposition onto such PV structures as copper indium gallium diselenide (CIGS), CdTe, and organic PVs.
Plasma deposition has been reported as convenient way for continuous TCO deposition of certain zinc oxide based TCO's at mild temperature (200-250° C.). The method is based on burning metalalkyl derivatives in oxygen plasma directed to substrate, so metal oxide fragments reach and are deposited on the substrate. Despite its obvious advantages the applicability of this approach to real PV structures has not been yet demonstrated.
For PV applications, parts of the TCO coating must be removed to form a required pattern (e.g., to form monolithically-integrated series-interconnected devices). This is normally accomplished by photolithography or laser ablation. Among the most widely used TCO materials, zinc oxide is one of the easiest materials to etch, tin oxide is the most difficult to etch, and indium oxide is intermediate in etching difficulty. Photolithography is a slow multi-step process, and laser ablation is not suitable for high throughput, large area manufacturing.
Solution-based preparation of different parts of PV devices (amorphous silicon layer, CIGS layer, organic PVs, CdS layer, TCO has recently been the subject of intensive research. Continuous wet manufacturing of TCO would complement roll-to-roll fabrication of PV devices. While noticeable progress has been achieved by both academia and industry in high-throughput fabrication of various PV components, suitable solution-based low-temperature TCO production remains a challenge and may be the ultimate barrier to fully solution-processed PVs. Solution processed TCO may additionally benefit solar cells based on CIGS, which surface is rough and full of crevices. Sputtering TCO on such a surface does not smoothen the surface, and does not close up the morphology defects. As a result, such structures are vulnerable to moisture penetration during the operation, which causes the cell degradation. A convenient wet process might be potentially useful for solving this problem.
Though ITO can be used for CIGS, aluminum doped zinc oxide (AZO) work function better fits the needs of the CIGS cell. AZO layer can be fabricated by deposition of metal acetate and/or alcoholate solution in toluene-isoproanol mixture followed by drying and decomposition to metal oxides at 450-500° C. Such a high process temperature is harmful for the CIGS layer. The process is done in air and thus does not provide required control of oxygen content. Necessary activation by additional heating in vacuum is sometimes also reported. Besides the above, other disadvantages of this approach are: poor surface wetting by metal derivatives employed, and film porosity caused by specifics of zinc oxide crystallization kinetics. As a result, multiple depositions are required to achieve acceptable performance.
US2008319143 and US20080319212 report an alternative approach consisting in application of metal siloxanolates of general formula M(OSiMe2OSiMe3)n (M(DM)n). Due to the liquid nature of these compounds and their structural similarity to silicones, their application obviously solves the problem of substrate wetting. It has been shown that spin coating solutions of Ti, V, Al, and Sn derivatives on glass and metal substrates leaves continuous liquid films, which are then converted into smooth solid films upon heating in humid air. Low electrical conductivity and doping abilities along with high optical transparency have been demonstrated for titania-vanadia compositions.